Video motion detection (VMD) is frequently used in the current security industry for detecting intruders entering a site, with predetermined scenes of interest. VMD is performed by computer software on a sequence of digital images captured by a video camera that is monitoring a particular scene of interest. Each image in the sequence is composed of an array of picture elements (pixels). Targets such as intruders typically show up in an image as a different intensity to the background scene in a captured image. VMD uses this in order to detect intruders in the sequence by looking for changes in pixel intensities that are consistent with a target moving through the scene. Groups of pixels associated with a target are also tracked from frame to frame to determine the direction of motion. If the scene is calibrated, the size of the target, the distance it has traveled, and the speed of its travel can be estimated from the tracked group of pixels. By ignoring targets that do not meet size, speed, and distance traveled criteria, the related security systems can be tuned to detect human and vehicle movement while rejecting small animal and foliage movement. Furthermore, a current VMD system is typically set up by defining which parts of the scene are to be sensitive to movement. By specifying large areas of the scene as sensitive, a highly effective detection system can be established in which intruders may be detected anywhere within those detection areas. Careful placement of these detection areas can also avoid false alarms from areas of no interest, such as car-parks or streets adjacent an area of interest.
One of the drawbacks of existing VMD systems is that existing systems are sensitive to all changes of intensity in the scene of interest. It is therefore possible to potentially detect the shadows and headlights of moving vehicles. If these shadows or headlight beams enter a detection area, and their movement meets the detection criteria, such movement will be wrongly detected as targets, even if the vehicles themselves are outside the detection area. This type of false alarm commonly occurs when vehicles travel down a road that is outside of, but runs parallel to, the boundary of an area being protected.
In more recent VMD systems, trigger lines have been adopted to reduce false alarms. With these systems the target has to cross the trigger line to cause an alarm, which reduces the likelihood of a false alarm by the specificity of the line placement. However, this detection method may also be undesirable because the trigger line must be carefully placed to ensure that all possible real target trajectories will cross the trigger line. Furthermore, in non-ideal conditions, it is possible that the target track will be lost and restarted as the target moves through the scene. If the target crosses the trigger line while not being tracked, no alarm will be raised. To reduce the effect of this possibility multiple, parallel trigger lines are often needed. Irrespective, the use of single or multiple trigger lines also does not prevent false alarms caused by shadows and headlight beams because these target tracks may still cross the necessary trigger lines due to the way the beams or shadow changes as the vehicle passes adjacent to the area of interest.
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.